To cope with the increasing shortage in frequency spectrum due to the proliferation of wireless services, cognitive radio has been considered as an attractive technique to improve spectrum utilization for future wireless systems. In cognitive radio networks, primary users may have priority in using a given radio channel. Secondary users can access a ratio channel in such a way that they cause little performance degradation to the primary users. Therefore, secondary users need to decide the presence or absence of a primary user. Previous attempts at detecting the usage of a channel by one or more primary users have had limited performance, especially in low bandwidth contexts.